Just adding: Undead Jesters are quite strong compared to Grynch Clan Goblins, so if you want to get them down as a level 30 with GFB or thunderstorm, you gonna need quite a few rounds of runes to defeat them. They can use a crazy teleport power and avoid your last blow on them with that.
Frankly, try to use your best char against them. Pay attention that they do not spawn with extremely large packs as the Grynch Clan Goblins do!
3 kills will be enough to add precious bestiary points (15) to your charm score.